Circuit board assembly employing solder vent hole

ABSTRACT

A printed circuit board assembly employing a solder vent hole adjacent solder filled interconnect vias connecting to a conductive pallet, is disclosed. The solder vent hole allows gases to escape from an otherwise sealed cavity during solder reflow, relieving positive pressure and thereby allowing solder to flow into it. By providing an escape path for trapped air and gases generated during solder paste reflow, the out-gassing pressure and weight of the molten solder is sufficient to allow the solder paste to flow into the cavity.

RELATED APPLICATION INFORMATION

This patent application claims the benefit pursuant to 35 USC §119(e) ofthe priority date of U.S. Provisional Patent Application Ser. No.60/472,710, filed on May 22, 2003, the entire contents of which arehereby expressly incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to printed circuit board assemblies andmethods for assembling electronic components into multi-layerassemblies. More particularly, the present invention relates to RF poweramplifier circuit board assemblies and related methods of assembly.

2. Description of the Prior Art and Related Background Information

In high power electronics assemblies generating a relatively high amountof heat, it is typically necessary to attach a printed circuit board(PCB) containing electronic components to a metal substrate provided asa heat sink. RF power amplifiers are an example of such electronicsmodule assemblies that incorporate heat dissipation means by design. Themetal substrate for an electronics module may further be needed forgrounding and electromagnetic interference (EMI) shielding. The use ofnonconductive adhesives is a particular cost effective method ofattaching a two sided printed circuit board to a metal substrate. Thedrawback to this approach is that there is no intimate metal-to-metalcontact between the metal substrate and the metalized backside of thePCB. To achieve electrical and thermal contact at critical locations ofthis interface, it is desirable to use surface applied solder paste thatis applied to a top surface of the PCB. However, trapped air and gasescreated during the solder reflow process create backpressure and preventthe molten solder from flowing.

Accordingly, a need presently exists for a way to use cost effectivenonconductive adhesive bonding and also provide effective electrical andthermal coupling to the substrate using surface applied solder reflowtechniques.

BRIEF SUMMARY OF THE INVENTION

In a first aspect the present invention provides an electronics assemblycomprising a printed circuit board having a via hole therethrough and anadjacent vent hole, a nonconductive adhesive layer having a firstsurface and a second surface, the first surface coupled to the printedcircuit board, and a conductive substrate coupled to the adhesive layersecond surface so that the adhesive layer is disposed between theprinted circuit board and the substrate. The adhesive layer further hasa void space overlapping the via hole and the vent hole. Reflowed solderis provided extending into the void space, the reflowed solderconnecting the printed circuit board to the substrate.

In a preferred embodiment of the electronics assembly a plurality ofelectronic devices are mounted on the printed circuit board. Thesubstrate is preferably made from an electrically conductive materialand the substrate provides electrical grounding for one or more of theelectronic devices on the printed circuit board through the reflowedsolder. A metalized pad may be provided plating and surrounding the viahole or vent hole. The substrate is also preferably made from athermally conductive material and provides heat dissipation for theprinted circuit board through the reflowed solder. For example, thesubstrate may be made from copper. The nonconductive adhesive layer maybe composed of a thermal set epoxy.

According to another aspect the present invention provides a method ofinterconnecting a printed circuit board, having a via hole and a venthole, and a conductive substrate. The method comprises coupling thesubstrate to the printed circuit board using a nonconductive adhesivelayer, the adhesive layer having a space forming a cavity between thesubstrate and the printed circuit board aligned to the via hole and venthole. The method further comprises reflowing solder from a top surfaceof the printed circuit board through the via hole to electricallyconnect the printed circuit board to the conductive substrate whilerelieving pressure in the cavity by venting gases in the cavity throughthe vent hole in the printed circuit board.

The method of interconnecting a printed circuit board to a substrate mayfurther comprise plating the via hole or vent hole before the reflowingof the solder. The substrate is preferably made from a metal materialand the substrate provides electrical grounding and heat dissipation forthe printed circuit board through the reflowed solder. The adhesivelayer is preferably an epoxy layer. For example, the epoxy layer may bea thermal set epoxy and the method comprises curing the epoxy at anelevated temperature.

According to another aspect the present invention provides an RF poweramplifier circuit board assembly comprising a printed circuit boardhaving a via hole therethrough, an adjacent vent hole therethrough, andone or more RF power transistors mounted thereon. The assembly furthercomprises a nonconductive adhesive layer having a first surface and asecond surface, the first surface coupled to the printed circuit board.An electrically conductive substrate is coupled to the adhesive layersecond surface so that the adhesive layer lies between the printedcircuit board and the substrate, the adhesive layer having a void spacealigned to the via hole and vent hole. Reflowed solder extends throughthe via hole and the void space to the substrate, the reflowed solderelectrically connecting the printed circuit board to the substrate. Oneor more conductive traces are provided on the printed circuit boardelectrically connecting to the reflowed solder.

The RF amplifier circuit board assembly may further comprise a metalizedpad plating and surrounding the via hole or the vent hole. The substrateis preferably composed of metal and the substrate provides electricalgrounding and heat dissipation for the printed circuit board through thereflowed solder.

According to another aspect the present invention provides a method ofassembling and electrically coupling a metal substrate to an RF printedcircuit board. The method comprises providing a printed circuit boardhaving a via hole, a vent hole adjacent the via hole, one or more RFpower transistors and one or more conductive traces. The methodcomprises providing a conductive metal substrate and a nonconductivelayer between the printed circuit board and substrate, the nonconductivelayer having an opening below the via hole and vent hole thereby forminga cavity. The method further comprises surface applying solder paste tothe via hole and heating and reflowing the solder paste from the topsurface of the printed circuit board through the via hole and opening inthe nonconductive layer to electrically connect the conductive trace tothe conductive metal substrate while relieving pressure in the cavity byventing gases in the cavity through the vent hole.

In a preferred embodiment of the method of assembling and electricallycoupling a conductive substrate to an RF printed circuit board, theconductive metal substrate is made from copper. The nonconductive layerpreferably comprises an epoxy layer and the method comprises bonding theprinted circuit board to the conductive substrate using thenonconductive epoxy layer. The conductive trace may be an RF groundconnection.

These and further aspects and features of the invention will beappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of an electronics assembly inaccordance with a preferred embodiment of the present invention showingindividual layers;

FIG. 2 is a top view of an interconnect portion of the electronicsassembly of FIG. 1;

FIG. 3 is a cross-sectional view of the interconnect portion as seenalong sectional line 3-3 of FIG. 2; and

FIG. 4 is an exemplary RF printed circuit board assembly in accordancewith the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is an exploded perspective view of an electronics assembly 10 inaccordance with a preferred embodiment of the present invention. In thisexample, assembly 10 comprises printed circuit board (PCB) layer 12,nonconductive adhesive layer 14, and a conductive support substrate orpallet 16. Pallet 16 is composed of a good electrical and thermalconductor, preferably a metal such as copper. Printed circuit boardlayer 12 is attached to the conductive substrate 16 using thenonconductive adhesive 14 (as shown in FIG. 3). For example, a thermalset sheet epoxy 14 may be employed. The use of a thermal setnonconductive adhesive sheet (or preform) to attach a printed circuitboard to a metal substrate is disclosed in U.S. Pat. No. 6,421,253 toDaniel Ash, Jr., issued Jul. 16, 2002, the disclosure of which isincorporated herein by reference in its entirety. The PCB layer 12includes various active and passive electronic components (showngenerally in FIG. 4) and in one preferred embodiment may be a two sidedPCB having electronic components and/or conductive traces on both sides(as described in more detail in the '253 patent). Some of thesecomponents will require electrical and/or thermal contact to thesubstrate. More particularly, in one preferred embodiment the assembly10 is adapted for high power electronics applications, such as RF poweramplifiers, and PCB layer 12 may further comprise a number of RF modulesand discrete components. Such power amplifier components may includeoutput couplers, power resistors, active amplifier devices (such asbipolar or LDMOS power transistors), EMI shielding (such as a groundplane in the circuit board, a shielding wall or lid), and othercomponents well known to those skilled in the art.

The assembly 10 includes layer interconnect portions 20 which providethe desired electrical and/or thermal contact of the PCB layer 12 to thepallet 16. These interconnect portions comprise via holes 22 forelectrically and/or thermally interconnecting layers 12 and 16 andadjacent vent holes 32 (interconnect portions 20 are illustrated indetail in FIGS. 2 and 3). The layers 12 and 14 are both illustrated intwo sections providing respective spaces 30, 38 which allow mounting ofcertain components such as power transistors, directly on the pallet 16for better thermal dissipation. Single piece layers 12 and 14 may alsobe employed, however, with the interconnect portions 20 providing thethermal coupling to pallet 16 or the layers 12 and 14 may have openingsfor mounting selected components directly on the pallet. The layers 12,14, 16 may also have additional mounting holes 18 for structurallyconnecting the layers or coupling the pallet to a heat sink and/or to alid. Adhesive layer 14 further includes void spaces 24 aligned with theinterconnect portions 20. As shown in FIGS. 1 through 4, void spaces 24form a cavity in the electronics assembly 10 under both the via holes 22and vent holes 32 through to pallet 16.

Referring to FIGS. 2 and 3 the layer interconnect portions 20 will bedescribed in more detail. FIG. 2 is a top view of one of the layerinterconnect portions 20 of electronics assembly 10. Cavity 24 isreferenced by the dashed lines as physically beneath PCB layer 12. FIG.3 is a cross sectional view of the interconnect portion 20, taken alongsectional line 3-3.

As shown in FIGS. 2 and 3, the electrical and/or thermal contact to thesubstrate 16 is provided by metalized pad(s) 28 with plated via holes 22and surface applied reflowed solder 26. The metalized pads 28 areelectrically coupled to traces on the PCB layer and/or are configuredadjacent to and in electrical and/or thermal contact with selectedelectronic components on the PCB layer 12 (as generally shown in FIG.4). As illustrated in FIG. 3, after assembly and curing the epoxy layer14 has a first surface 34 coupled to the printed circuit board layer 12and a second surface 36 coupled to the substrate 16 so that the epoxylayer is disposed between the printed circuit board 12 and thesubstrate. As shown, spaces 24 in the nonconductive adhesive sheet 14are provided for making contact between the printed circuit board andthe metal substrate with the reflowed solder 26. The spaces 24 in theepoxy sheet 14 would normally create air tight voids 24 as may be seenfrom FIG. 3, which would prevent surface applied solder 26 from flowinginto the hole 22 to make adequate contact with the substrate 16 due toincreased pressure of air trapped in the void as the solder is heated.Other gases and further backpressure may be created by the reflowedsolder itself. To address this problem, vent hole 32 is provided in thePCB layer 12 adjacent to each via 22 and extending into void 24. The viaholes 22 and vent holes 32 may be provided in the PCB 12 through any ofa variety of known techniques, including drilling, etching or duringformation of the PCB material. The vent hole 32 may be provided eitherbefore or after assembly with the substrate 16, prior to solder reflowprocessing, and may preferably be formed together with via hole 22 tofacilitate alignment of the two holes in a closely spaced configuration.The vent hole 32 very effectively relieves gaseous backpressure in thevoid 24, thereby allowing applied solder paste 26 to flow down throughthe vias 22 and connect the printed circuit board 12 to the substrate 16as shown. Alternatively, the metal plating 28 may be applied to venthole 32. In this approach the solder paste is applied to via hole 22 asbefore and upon heating flows down into void 24 and flows across thevoid to the vent hole to contact the plating 28 and electrically andthermally connect plating 28 and substrate 16. In this embodiment thevent hole rather than the via hole acts as the electrical and/or thermalconnection point. (It will be readily appreciated that FIG. 3 equallyillustrates this alternate structure with reference numeral 22 nowindicating the vent hole and reference numeral 32 the via hole, althoughin this embodiment the solder 26 may be localized mostly in the void 24and not extend as far up toward the top surface of PCB layer 12 as isshown in FIG. 3). In this embodiment the via hole 22 may be of largerdiameter than the vent hole 32 to facilitate the solder flow and the twoholes are preferably very closely spaced. Also, the gas venting assistsin the solder flow and this embodiment may enhance this effect making ita preferred approach for some applications. Therefore, in eitherembodiment due to vent hole 32, the desired good electrical and/orthermal contact to the substrate 16 is provided by the reflowed solder.

In FIG. 4, an exemplary RF power amplifier circuit board assembly 40 ofthe present invention is illustrated. The illustrated assembly is highlyschematic in nature as specific implementations will have a variety oflayouts and components, as will be appreciated by those skilled in theart. The RF power amplifier circuit board assembly 40 comprises aprinted circuit board 12 having interconnect portion(s) 20 (asillustrated in FIGS. 2 and 3). Further, RF power amplifier circuit boardassembly 40 has one or more RF power transistors 44 and various otherdiscrete components 42, 43 mounted thereon. The RF power amplifiercircuit board also includes conductive circuit traces 46 thatinterconnect discrete components and also provide a ground connection,as explained herein, to the pallet 16 through interconnect portions 20.Conductive traces 46 are shown on the top surface of PCB layer 12 inFIG. 4, for ease of illustration, but traces may also be configured onthe bottom surface of PCB layer 12 as described in more detail in the'253 patent. A ground plane layer may also be provided within layer 12and this may also connect to ground traces 46 and the pallet 16 throughinterconnect portions 20. Also, interconnect portions 20 may be disposedadjacent selected electronic components to provide thermal coupling tothe pallet 16 for “hot spots”, even where electrical connections are notneeded (as illustrated generally by component 43 and adjacentinterconnect portion 20).

The invention as illustrated herein additionally includes a method ofassembling and interconnecting a PCB to a conductive substrate. Themodifications to the method of assembly described in the above noted'253 patent will be apparent from the above. A preferred embodiment ofthis method comprises assembling PCB 12 and substrate 16 using anonconductive adhesive layer 14, followed by reflowing solder throughthe via hole 22 in the PCB and the cavity 24 below the via hole 22. Morespecifically the method comprises applying solder paste 26 to the viahole 22 from above using conventional techniques and heating the solderpaste 26 to a temperature to provide reflow of the solder paste into thevia hole. The vent holes 32 assist the reflow process by venting gasesin the cavity 24 and relieving backpressure as described above. Thesolder paste thus flows from the upper surface of the printed circuitboard 12 through the via hole 22 into cavity 24 to electrically connectto the conductive substrate 16 while substantially maintaining equalizedpressure through the venting of the cavity 24. Other aspects andfeatures of the method of the invention may incorporate additionalteachings of the '253 patent. For example, details on the provision of asuitable nonconductive adhesive layer 14, such as a thermal set epoxyperform, and bonding of the layers using the adhesive, are disclosed inthe '253 patent and may be employed herein, as will be readilyappreciated by those skilled in the art.

While various embodiments of the invention have been described, it willbe apparent to those of ordinary skill in the art that many moreembodiments and implementations are possible that are within the scopeof this invention. It is understood that such changes in the specificstructure and method shown and described may be made within the scope ofthe claims, without departing from the spirit of the invention. Also,nothing herein is intended to limit or waive the full scope of themeaning of the claims or limit in any way the scope of equivalents ofthe claims under the judicial doctrine of equivalents.

1. An electronics assembly, comprising: a printed circuit board having avia hole therethrough and an adjacent vent hole; a nonconductiveadhesive layer having a first surface and a second surface, the firstsurface coupled to the printed circuit board; a conductive substratecoupled to the adhesive layer second surface so that the adhesive layeris disposed between the printed circuit board and the substrate, whereinthe adhesive layer further has a void space overlapping the via hole andthe vent hole; and reflowed solder extending into the void space, thereflowed solder connecting the printed circuit board to the substrate.2. The electronics assembly of claim 1, further comprising a pluralityof electronic devices mounted on the printed circuit board.
 3. Theelectronics assembly of claim 1, further comprising a metalized padplating and surrounding the via hole.
 4. The electronics assembly ofclaim 1, wherein the substrate is made from an electrically conductivematerial and wherein the substrate provides electrical grounding for oneor more of the electronic devices on the printed circuit board throughthe reflowed solder.
 5. The electronics assembly of claim 1, wherein thesubstrate is made from a thermally conductive material and provides heatdissipation for the printed circuit board through the reflowed solder.6. The electronics assembly of claim 4, wherein the substrate is madefrom copper.
 7. The electronics assembly of claim 5, wherein thesubstrate is made from copper.
 8. The electronics assembly of claim 1,wherein the nonconductive adhesive layer is composed of a thermal setepoxy. 9-13. (canceled)
 14. An RF power amplifier circuit boardassembly, comprising: a printed circuit board having a via holetherethrough, an adjacent vent hole therethrough, and one or more RFpower transistors mounted thereon; a nonconductive adhesive layer havinga first surface and a second surface, the first surface coupled to theprinted circuit board; an electrically conductive substrate coupled tothe adhesive layer second surface so that the adhesive layer liesbetween the printed circuit board and the substrate, wherein theadhesive layer further has a void space aligned to the via hole and venthole; reflowed solder extending through the via hole and the void spaceto the substrate, the reflowed solder electrically connecting theprinted circuit board to the substrate; and one or more conductivetraces on the printed circuit board electrically connecting to thereflowed solder.
 15. The RF amplifier circuit board assembly of claim14, further comprising a metalized pad plating and surrounding the viahole.
 16. The RF amplifier circuit board assembly of claim 14, whereinthe substrate is composed of metal and wherein the substrate provideselectrical grounding and heat dissipation for the printed circuit boardthrough the reflowed solder. 17-20. (canceled)